Now this is all just speculation as I'm no engineer, but I'd think the Sony system in this car would drain the battery faster than a traditional car stereo of yore. As the Sony system contains MFT, you'd be running the computer that powers that system as well as that large touch screen. As others have said, even if the fans were off, it is possible that the computers for the climate control system were running. If you happened to have the door open for a period of time you'd make things even worse as the fuel pump primes itself when the drivers door is open. Oh and the Sony system has a subwoofer, those things are hungry.